The present invention relates to a novel light absorbing polymer which may be useful in instances where light absorption is advantageous, particularity in antireflective coating compositions or as light absorbing additives. Antireflective coatings are used in image processing by forming either a thin layer between a reflective substrate and a photoresist coating or as a dyed coating on top of a photoresist. Such compositions are especially useful in the fabrication of semiconductor devices by photolithographic techniques. The polymer may also be used as an additive in photoresists to prevent reflection of light from the substrate.
Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a thin coating of film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure to radiation.
This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the photoresist.
The trend towards the minitiarization of semiconductor devices has lead to the use of sophisticated multilevel systems to overcome difficulties associated with such minitiarization. The use of highly absorbing antireflective coatings in photolithography is a simpler approach to diminish the problems that result from back reflection of light from highly reflective substrates. Two deleterious effects of back reflectivity are thin film interference and reflective notching. Thin film interference results in changes in critical linewidth dimensions caused by variations in the total light intensity in the resist film as the thickness of the resist changes. Variations of linewidth are proportional to the swing ratio (S) and therefore must be minimized for better linewidth control. Swing ratio is defined by: EQU S=4(R.sub.1 R.sub.2).sup.1/2 e.sup.-.alpha.D
where R.sub.1 is the reflectivity at the resist/air or resist/top coat interface, PA1 where R.sub.2 is the reflectivity at the resist/substrate interface, PA1 where .alpha. is the resist optical absorption coefficient, and PA1 D is the film thickness. PA1 X is N.dbd.N, R'C.dbd.C R', R'C.dbd.N or N.dbd.C R', where R' is H or alkyl; PA1 Y is aryl, aralkyl, heterocyclic or alkyl; PA1 m=1-3; and PA1 at least one nonaromatic comonomer unit of the formula: ##STR2## where, R.sub.6 -R.sub.9 are independently hydrogen, alkyl, alkoxy, halo, alkyl ether, amino, hydroxy, fluoroalkyl, amide, alkyl amide, carboxylic acid, carboxylic ester, sulfonic acid, sulfonic ester, alkali salts of carboxylic acid and sulfonic acid, crosslinking group or R.sub.8 and R.sub.9 combined to form an anhydride group. PA1 X is N.dbd.N, R'C.dbd.C R', R'C.dbd.N or N.dbd.C R', where R' is H or alkyl; PA1 Y is aryl, aralkyl, heterocyclic or alkyl; PA1 m=1-3.
Bottom antireflective coatings function by absorbing the radiation used for exposing the photoresist, thus reducing R.sub.2 and thereby reducing the swing ratio. Reflective notching becomes severe as the photoresist is patterned over substrates containing topographical features, which scatter light through the photoresist film, leading to linewidth variations, and in the extreme case, forming regions with complete resist loss. Similarily, dyed top antireflective coatings reduce the swing ratio by reducing R.sub.1, where the coating has the optimal values for refractive index and absorption characteristics, such as absorbing wavelength and intensity.
In the past dyed photoresists have been utilized to solve these reflectivity problems. However, it is generally known that dyed resists only reduce reflectivity from the substrate but do not substantially eliminate it. In addition, dyed resists also cause reduction in the lithographic performance of the photoresist, together with possible sublimation of the dye and incompatibility of the dye in resist films. In cases where further reduction or elimination of the swing ratio is required, the use of a top or bottom antireflective coating provides the best solution for reflectivity. The bottom antireflective coating is applied to the substrate prior to coating with the photoresist and prior to exposure. The resist is exposed imagewise and developed. The antireflective coating in the exposed area is then etched, typically in an oxygen plasma, and the resist pattern is thus transferred to the substrate. The etch rate of the antireflective film should be relatively high in comparison to the photoresist so that the antireflective film is etched without excessive loss of the resist film during the etch process. The top antireflective coating is applied on top of a film of photoresist. The system is then imagewise exposed and developed to give a pattern on the substrate. Top antireflective coatings are disclosed in EP 522,990, JP 7,153,682, JP 7,333,855 and in pending patent application with U.S. Ser. No. 08/811,807 filed on Mar. 6, 1997, based on provisional application 60/013,007 filed on Mar. 7, 1996, now abandoned, and all incorporated herein by reference.
Antireflective coatings containing a dye for absorption of the light and an organic polymer to give coating properties are known. However, the possibility of sublimation and diffusion of the dye into the environment and into the photoresist layer during heating make these types of antireflective compositions undesirable.
Polymeric organic antireflective coatings are known in the art as described in EP 583,205, and incorporated herein by reference. However, the antireflective films as disclosed in EP 583,205 are cast from organic solvents, such as cyclohexanone and cyclopentanone. The potential hazards of working with such organic solvents, have lead to the development of the antireflective coating composition of the instant invention, where the solid components of the antireflective coating are both soluble and spin castable from solvents having lesser toxicity hazards. The preferred solvents that are known in the semiconductor industry to have low toxicity among others are propylene gycol monomethyl etheracetate (PGMEA), propylene gycol monomethylether (PGME), and ethyl lactate (EL). An even more preferred solvent is water for its ease of handling and transportation. The antireflective coating of the present invention can be cast from these low toxicity solvents, water, or mixtures of water and lower alcohols, ketones or esters that are miscible in water. Antireflective coatings are also disclosed in U.S. Pat. No. 5,525,457 and in pending patent applications with U.S. Ser. Nos. 08/698,742 filed on Aug. 16, 1996, 08/699,001 filed on Aug. 16, 1996, and 08/724,109 filed on Sep. 30, 1996, and all incorporated herein by reference. However, the novel dye functionality of the instant invention when attached to the specific types of monomer described, makes the instant invention significantly different from the prior art referred to previously. Another advantage of using antireflective coatings soluble in the prefered, lower toxicity solvents, is that these same solvents can also be used to remove the edge bead of the antireflective coating and no additional hazards or equipment expense is incurred, since these solvents are also used for photoresist and photoresist processing. The antireflective coating composition also has good solution stability. Additionally, substantially no intermixing is present between the antireflective coating and the photoresist film. The antireflective coating also has good dry etching properties, which enable a good image transfer from the resist to the substrate and good absorption characteristics to prevent reflective notching and linewidth variations.